Glow plug for internal combustion engine

ABSTRACT

A glow plug has a heating coil made of an alloy of iron, chromium and aluminum, a control coil made of nickel and serially connected with the heating coil, a heater case made of an inconel material and accommodating the coils therein, and a current line through which an electric current adjusted at a duty ratio changeable based on a voltage of the current according to a pulse width modulation control is supplied to the coils. The alloy of the heating coil and nickel of the control coil are superior in durability at high temperature. The inconel material of the case is superior in heat resisting property. In response to the current, resistance of the control coil is heightened, and the heating coil is heated to a maximum temperature to heat an air-fuel mixture of a combustion chamber of a diesel engine.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of the prior Japanese Patent Application 2006-343829 filed on Dec. 21, 2006 so that the contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a glow plug used to preheat an air-fuel mixture in a combustion chamber of an internal combustion engine such as a diesel engine of a vehicle.

2. Description of Related Art

A glow plug is, for example, attached to an internal combustion engine such as a diesel engine of a vehicle to preheat a compressed air-fuel mixture in a combustion chamber of the engine. Published Japanese Patent First Publication No. H11-281059 (1999) discloses a self-temperature control type glow plug disposed to be projected into a combustion chamber of a diesel engine intermittently filled with an air-fuel mixture. This plug has both a control coil and a heating coil made of different materials and serially connected to each other. Each of the coils has a positive temperature coefficient of resistance. That is, as the temperature of each coil is heightened, a resistance value of the coil is increased. The coefficient in the control coil is set to be larger than that in the heating coil.

When an electric current set at a direct voltage is continuously supplied from a battery of a vehicle to the coils, with time the temperature in each coil rises. In response to this temperature rise, the resistance of the control coil having a larger temperature coefficient of resistance becomes large with time. Therefore, a level of the current flowing through the heating coil is lowered with time, and the temperature of the heating coil is saturated at a certain value. This saturated temperature is called a maximum temperature. An air-fuel mixture of the combustion chamber is preheated by the heating coil heated at the maximum temperature.

In this operation of the plug, the maximum temperature reachable in the glow plug and a temperature rise of the plug are changed with a voltage of the battery, and the battery voltage is easily changed. Therefore, it is required to control the temperature of the glow plug according to the battery voltage. In this case, it is difficult to heat the glow plug at high speed or to heighten the maximum temperature of the glow plug. Further, the battery voltage is largely changed with operating conditions of a starter or an alternator or the circumference temperature. For example, in a cranking operation using the starter, the battery voltage is largely dropped due to the driving of an electric motor, so that it is difficult to control the temperature of the glow plug.

As described above, in the conventional glow plug, the temperature rise of the glow plug at the cranking is delayed. Further, it is difficult to control the maximum temperature of the glow plug to an appropriate value or to continue the glow plug set at the maximum temperature for an appropriate time. As a result, startability of the engine is degraded, and it becomes difficult to suppress the generation of white smoke in a cold engine condition. Further, it is desired to compress an air-fuel mixture at a low compression ratio for the purpose of obtaining a low emission vehicle. However, because it is difficult to sufficiently heighten the temperature of the air-fuel mixture by the glow plug, there is a high probability that the engine may require an air-fuel mixture compressed at a high compression ratio.

To solve these problems, there is a technique that a quantity of electric power supplied to the coils is controlled according to a pulse width modulation (PWM) current control. In this technique, the temperature of the glow plug can be adequately controlled without receiving an adverse influence of a change in the battery voltage. Therefore, the glow plug can quickly be heated, and the glow plug can reliably reach a maximum temperature expected in advance. That is, the maximum temperature designed in advance can substantially be heightened in the glow plug.

When the glow plug is heated to a higher maximum temperature at a high speed under the PWM current control, it is necessary for the control coil, the heating coil and a heater case accommodating the coils to have an excellent heat resisting property. However, in the conventional glow plug, the heating coil is made of nickel-chromium (Ni—Cr) alloy, the control coil is made of cobalt-iron (Co—Fe) alloy, and the heater case is made of stainless steel. Therefore, it is difficult for the heating coil or the heater case to have a sufficient level of durability at a high temperature.

SUMMARY OF THE INVENTION

An object of the present invention is to provide, with due consideration to the drawbacks of the conventional glow plug, a glow plug of which a temperature is easily controlled and which has an excellent durability at high temperature.

According to an aspect of this invention, the object is achieved by the provision of a glow plug comprising a heating coil made of an alloy of iron, chromium and aluminum, a control coil made of nickel and serially connected with the heating coil, a current supply line through which an electric current adjusted according to a pulse width modulation control is supplied to the control coil and the heating coil to heat the heating coil to a maximum temperature while heightening a resistance of the control coil, and a heater case, made of an inconel material, that accommodates the heating coil and the control coil so as to dispose the control coil on a rear side of the heating coil and insulates the heating coil and the control coil from a heated medium disposed outside the heater case.

With this structure of the glow plug, temperatures of the control coil and the heating coil receiving the electric current through the current supply line are heightened. Further, because the control coil is made of nickel which has a positive temperature coefficient of resistance, a resistance of the control coil is increased as the temperature of the control coil is heightened. In this case, the electric current supplied to the heating coil is decreased as the temperature of the heating coil is heightened, and the temperature of the heating coil is saturated. That is, the heating coil is maintained at a maximum temperature. A heater disposed outside the heater case is heated by the heating coil through the heater case.

Further, the electric current is adjusted at a duty ratio changeable based on a voltage of the electric current according to the pulse width modulation control, so that an amount of electric power supplied to the coils is adjusted so as not to receive an influence of a change in the voltage of the electric current. Therefore, the temperatures of the control coil and the heating coil can be easily controlled without receiving an influence of a change in the voltage of the electric current, and the heating coil can reliably be heated to the maximum temperature at a high speed. Accordingly, a delay in the heating of the heated medium can be suppressed, and the glow plug can reliably continue heating the heated medium at the maximum temperature for a predetermined time. For example, when the glow plug is used to heat an air-fuel mixture of a combustion chamber of an internal combustion engine, startability of the engine can be improved, and a white smoke generated in a cold engine operation can be reduced.

Moreover, because the heating coil is made of an alloy of iron, chromium and aluminum, the heating coil can have an excellent durability at a high temperature. Accordingly, even when the heating coil is repeatedly heated to the maximum temperature such as 1000° C. according to the pulse width modulation control for a long use of the plug, the heating coil can maintain a high heat resisting property for a long time.

Furthermore, because the control coil is made of nickel, a control coil of superior durability can be produced at a low cost as compared with a cobalt-iron alloy used for a conventional glow plug. Nickel has a positive temperature coefficient of resistance smaller than that of the cobalt-iron alloy. However, because the temperature of the control coil is adjusted according to the pulse width modulation control, the control coil is not required to have a large positive temperature coefficient of resistance.

Still further, because the heater case is made of an inconel material (trademark of Inco Alloys International, INC.), the heater case is superior in durability. Particularly, even when the heater case is repeatedly heated at a high temperature by the coils according to the pulse width modulation control for a long use of the plug, the heater case can reliably accommodate the coils without being broken.

Accordingly, because of the heating coil being made of an alloy of iron, chromium and aluminum, the control coil being made of nickel and the heater case being made of the inconel material, the temperature of the glow plug can be easily controlled, and the glow plug can have an excellent durability at a high temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view, partially in cross-section, of a glow plug according to the embodiment of the present invention;

FIG. 2 is a front view, with a portion broken away for clarity, of a front portion of the glow plug shown in FIG. 1;

FIG. 3 is a front view of the front portion of the glow plug shown in FIG. 1;

FIG. 4 is a perspective front view of a front portion of a conventional glow plug in a comparative example;

FIG. 5 is a front view of the front portion of the glow plug shown in FIG. 4;

FIG. 6 shows a relation between durability of a heating coil and a maximum temperature of the heating coil for each of materials in the heating coil;

FIG. 7 shows a relation between durability of a heating coil and a maximum temperature of the heating coil for each of materials in a heater case;

FIG. 8 shows a relation between a length of a control coil and a maximum temperature of a heating coil for each of three values of an applied voltage of a controlled current;

FIG. 9 shows a relation between a length of packed carbon particulate matters and a maximum temperature of a heating coil for each of lengths in a control coil;

FIG. 10 is a sectional view of a plug hole penetrating through a wall of a combustion chamber;

FIG. 11A is an explanatory view showing carbon particulate matters packed in the plug hole shown in FIG. 10 by a length of 10 mm;

FIG. 11B is an explanatory view showing carbon particulate matters packed in the plug hole shown in FIG. 10 by a length of 15 mm;

FIG. 11C is an explanatory view showing carbon particulate matters packed in the plug hole shown in FIG. 10 by a length of 20 mm;

FIG. 12A is an explanatory view showing carbon particulate matters packed in the plug hole shown in FIG. 10 by a length of 25 mm;

FIG. 12B is an explanatory view showing carbon particulate matters packed in the plug hole shown in FIG. 10 by a length of 30 mm;

FIG. 12C is an explanatory view showing carbon particulate matters packed in the plug hole shown in FIG. 10 by a length of 33 mm;

FIG. 13 shows a variation in resistance values of control coils caused by swaging;

FIG. 14 shows a relation between durability of a heating coil and a maximum temperature of the heating coil for each of diameters in a wire forming the heating coil; and

FIG. 15 shows a relation between durability of a heating coil and a maximum temperature of the heating coil for each of diameters in a wire forming a control coil.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present invention will now be described with reference to the accompanying drawings, in which like reference numerals indicate like parts, members or elements throughout the specification unless otherwise indicated.

Embodiment

FIG. 1 is a front view, partially in cross-section, of a glow plug according to this embodiment, FIG. 2 is a front view, with a portion broken away for clarity, of a front portion of the glow plug shown in FIG. 1, and FIG. 3 is a front view of the front portion of the glow plug shown in FIG. 1. A glow plug shown in FIG. 1 to FIG. 3 is attached to a diesel engine (not shown) representing an internal combustion engine such that a front portion of the plug is projected into each of a plurality of combustion chambers of the engine. In place of the combustion chamber, the plug may be projected into a pre-combustion chamber, a swirl chamber or the like.

As shown in FIG. 1 and FIG. 2, a glow plug 1 has a heater case 4 disposed on a front side of the plug 1, a heating coil 2 accommodated in a front side portion of the case 4, a control coil 3 serially connected with a rear end of the heating coil 2 and accommodated in the case 4, and a cylindrical housing 5 disposed on a rear side of the plug 1 so as to hold a rear end of the case 4. The heating coil 2 is made of an alloy of iron, chromium and aluminum (Fe—Cr—Al alloy). The control coil 3 is made of nickel (Ni) substantially set at 100 wt %. The heater case 4 is made of an inconel material (trademark of Inco Alloys International, INC.) which mainly contains nickel, slightly contains carbon, silicon, manganese, chromium and iron, and sometimes contains niobium, aluminum and titan. Each of the coils 2 and 3 has a positive temperature coefficient of resistance, and the coefficient in the control coil 3 is larger than that in the heating coil 2.

A length L1 of the heating coil 2 along an axial direction of the plug 1 is set within a range from 5 to 10 mm. A length L2 of the control coil 3 along the axial direction is preferably set within a range from 5 mm to 12 mm. A wire forming each of the coils 2 and 3 is preferably set to have a diameter ranging from 0.31 mm to 0.35 mm.

As shown in FIG. 3, the heater case 4 has a cap portion 40 on the front side opposite to the housing 5 so as to isolate the coils 2 and 3 from an air-fuel mixture compressed in the combustion chamber, so that the case 4 is formed almost in a U-shape in section. The case 4 further has a smaller diameter portion 41 disposed on the front side of the case 4, a larger diameter portion 43 disposed on the rear side of the case 4, a middle diameter portion 42 disposed between the portions 41 and 43, a first taper portion 441 connecting the portions 41 and 42, and a second taper portion 442 connecting the portions 42 and 43.

Each of the portions 41, 42 and 43 is formed in a cylindrical shape. The portion 41 has a diameter D1 set at 3.5 mm. The portion 42 has a diameter D2 set at 4.0 mm. The portion 43 has a diameter D3 set at 5.0 mm. A length A1 of a portion of the case 4 protruded from the housing 5 is set within a range from 28 to 36 mm. A length A2 of the portion 41 along the axial direction is set within a range from 16 to 24 mm.

As shown in FIG. 2, the whole coils 2 and 3 are disposed in an inner opening of the portion 41, so that a rear end 31 of the coil 3 is placed to be shifted from the taper portion 441 toward the front side. A front end of the coil 3 is connected with a rear end of the coil 2. A front end 21 of the coil 2 is connected with the cap portion 40 of the case 4, so that the coil 2 is grounded. Further, a front end 51 of the housing 5 is disposed so as to overlap with the second taper portion 442, so that the larger diameter portion 43 is held by the housing 5 so as to be attached to an inner wall of the housing 5.

As shown in FIG. 1, a powder layer 12 having a heat resisting property and an electric insulation property is packed into the case 4 to insulate the heating coil 2, the control coil 3 and the case 4 from one another. Powder particles made of magnesium oxide (MgO) form the powder layer 12. The powder layer 12 is packed so as to reach the rear end of the case 4, and the layer 12 is sealed with a rubber seal 13 at the rear end of the case 4.

As shown in FIG. 1, the plug 1 further has a pin holding element (or current supply line) 111 connected with the rear end 31 of the coil 3 and extending toward the rear side in an inner opening of the housing 5, and a screw pin (or current line) 112 connected with a rear end of the element 111 and extending toward the rear side in the inner opening of the housing 5. A rear end of the screw pin 112 is protruded from a rear end of the housing 5. The plug 1 further has a ring rubber 141 attached to a rear end of the pin 112 and a resin-made bush 142 attached to the ring rubber 141. An opening between the housing 5 and the pin 112 is occupied by the rubber 141 and the bush 142 such that the rubber 141 and the bush 142 insulate the housing 5 and the pin 112 from each other. A nut 143 is fastened to the housing 5 so as to fix the rubber 141 and the bush 142 to the housing 5. Further, a terminal nut 144 is screwed to the rear end of the pin 112, and the pin 112 is electrically connected with an external lead wire 8 through the nut 144. This lead wire 8 is connected with a pulse width modulation (PWM) driver (or current supply circuit) 9 disposed outside the plug 1, and an electric power is supplied from a positive electrode of a battery (not shown) to the PWM driver 9. The PWM driver 9 controls an amount of electric current supplied to the coils 2 and 3 according to a pulse width modulation (PWM) current control while considering a voltage of the battery. The PWM driver 9 supplies a controlled electric current to the control coil 3 and the heating coil 2 through the pin 112 and the element 111.

Next, an operation of the glow plug 1 is now described below. The PWM driver 9 produces an electric current at a controlled duty ratio set according to a PWM current control and supplies this current to the coils 2 and 3 as a controlled current. This controlled current is set at a direct voltage depending on a voltage of the battery.

Therefore, the voltage of the controlled current is changeable. In the PWM current control, a transistor is repeatedly turned on and off in a very short cycle, and a pulse-shaped electric current is transmitted to the coils 2 and 3 through the transistor set in an on state. The duty ratio is defined as a ratio of an on-state period to one cycle period. In response to this controlled current, the temperature in each coil is rised with time. In response to this temperature rise, the resistance of the coils 2 and 3 becomes large with time, so that the current flowing through the heating coil 2 is lowered with time. When the temperature of the coils 2 and 3 reaches a certain value, the temperature of the coils 2 and 3 is saturated. This saturated temperature is called a maximum temperature.

For example, when the battery voltage becomes low, the PWM driver 9 increases the duty ratio to maintain a predetermined change with time in the temperatures of the coils 2 and 3. In contrast, when the battery voltage becomes high, the PWM driver decreases the duty ratio to maintain the predetermined change with time. That is, the duty ratio is controlled such that the temperatures of the coils 2 and 3 are always heightened to the predetermined temperature change with time regardless of a change in the battery voltage. Therefore, although the battery voltage is changed largely in response to an operation of a starter or an alternator or is changed with a circumference temperature, the temperatures of the coils 2 and 3 can be heightened to a predetermined temperature change with time without receiving an adverse influence of a change in the battery voltage. As a result, the glow plug 1 can stably and reliably reach a maximum temperature originally set at a high speed. That is, the maximum temperature can originally be set at a higher value than that in a case where no PWM current control is used.

Accordingly, a delay in the heating of the glow plug 1 at the cranking can be suppressed, and the glow plug 1 can reliably continue heating the air-fuel mixture at the maximum temperature for a predetermined time. Further, startability of the engine can be improved, and the generation of white smoke in a cold engine condition can be reduced.

Further, the heating coil 2 is made of the Fe—Cr—Al alloy, so that the coil 2 has an excellent durability at a high temperature such as 1000° C. Therefore, even when the coil 2 is repeatedly heated to a high temperature according to the PWM current control for a long use of the plug 1, the heating coil 2 can maintain a high heat resisting property for a long time.

Moreover, because the control coil 3 is made of Ni, a control coil of superior durability is produced at a low cost as compared with that of a cobalt-iron alloy used for a conventional glow plug. Nickel has a positive temperature coefficient of resistance smaller than that of the cobalt-iron alloy, so that a self-temperature control performance for controlling the temperature of the coil 3 by itself is low. However, because the temperature of the control coil is adjusted according to the PWM current control, the control coil 3 is not required to have a large positive temperature coefficient of resistance. Accordingly, the coil 3 superior in durability can be produced at a low cost.

Furthermore, the heater case 4 is made of the inconel material, so that the case 4 is superior in heat resisting property. Therefore, even when the case 4 is repeatedly heated at a high temperature by the coils according to the PWM current control for a long use of the plug 1, the case 4 can reliably stably accommodate the coils 2 and 3 without breakage, melting or deformation of the case 4.

In conclusion, because the glow plug 1 is constituted by the heating coil 2 made of the Fe—Cr—Al alloy, the control coil 3 made of Ni and the heater case 4 made of the inconel material, the glow plug 1 can have a high heat resisting property appropriate to the PWM current control. Accordingly, the temperature of the glow plug 1 (i.e., temperatures of the coils 2 and 3) can easily be controlled, and the glow plug 1 can has an excellent durability at the high temperature.

As another feature of the plug 1, the length L2 of the control coil 3 is shortened within a range from 5 to 12 mm such that the rear end of the coil 3 is placed far away from an attaching area (i.e., area of the top end 51 of the housing 5) between the case 4 and the housing 5. In other words, the length A1 is set to be smaller than a sum of the lengths L1 and L2. With this structure, even when carbon particulate matters (or carbon soot) derived from unburned fuel are packed into an opening formed around the plug 1, the temperature change of the coil 3 can reliably be performed according to the PWM current control. The reason is as follows.

A plug hole is formed in a wall of a combustion chamber of the engine to receive the glow plug 1 such that a front portion of the case 4 projects into the chamber, and an opening is inevitably formed between the case 4 of the plug 1 and a wall surrounding the plug hole. When the plug 1 is used for a long time, the opening is stuffed or packed with the carbon particulate matters. The carbon particulate matters are gradually deposited on an outer wall of the case 4 while growing from the attaching area toward the front side.

Assuming that a rear end of a control coil is placed near the attaching area, the carbon particulate matters are easily deposited on an outer wall of a portion of a heater case surrounding the control coil. In this case, heat of the coil is dispersed through the carbon particulate matters, so that a predetermined relation between resistance of the control coil and electric power supplied to the coil is not satisfied. That is, even when a predetermined electric power is supplied to the coil to heighten the resistance of the coil at a predetermined value as designed, the resistance of the coil actually becomes lower than the predetermined value, and electric power is excessively supplied to a heating coil. Therefore, a current supplied to the heating coil cannot be adequately controlled, and the heating coil is sometimes disconnected or broken.

In contrast, in this embodiment, because the rear end of the control coil 3 is placed far away from the attaching area between the case 4 and the housing 5, the carbon particulate matters are hardly deposited on an outer wall of a portion of the case 4 surrounding the control coil 3. Therefore, the excessive heating of the heating coil 2 can be prevented. Accordingly, the PWM current control for supplying a controlled current to the coil 3 can be adequately performed without being disturbed by the carbon particulate matters.

Further, the length L2 of the control coil 3 is shortened so as to lower a current reduction property obtained by the increase of resistance with a temperature rise. For example, assuming that the control coil 3 has a length shorter than 5 mm, it is difficult to control the temperature of the heating coil 2. However, in this embodiment, the control coil 3 has the length L2 equal to or longer than 5 mm, and the current reduction using the control coil 3 is controlled according to the PWM current control. Therefore, the temperature of the heating coil 2 can be reliably adjusted.

As still another feature of the plug 1, the case 4 is formed so as to have three portions 41, 42 and 43, and these portions are shaped by swaging so as to have different diameters. Therefore, a difference in diameter between two portions adjacent to each other becomes small as compared with a case where a conventional heater case has two portions of different diameters. More specifically, to form the case 4, a pipe formed in a U-shape in section and having a diameter equal to the diameter D3 of the larger diameter portion 43 is prepared, the coils 2 and 3 are disposed in the pipe, and MgO powder is packed into the pipe. Then, a portion of the pipe is compressed by swaging to have a diameter equal to the diameter D2 of the middle diameter portion 42. Then, a front area of the compressed portion of the pipe is again compressed by swaging to have a diameter equal to the diameter D1 of the smaller diameter portion 41.

Assuming that a heater case has only a smaller diameter portion and a larger diameter portion and accommodates a control coil extending in both the portions, a difference in diameter between the portions becomes large. Therefore, a strong compressive force is required to form the smaller diameter portion by swaging. During the formation of the smaller diameter portion, a portion of the control coil placed in the smaller diameter portion also receives the strong compressive force, and there is also a high probability of a wire forming the portion of the control coil being undesirably thinned so as to increase an electrical resistivity of the coil. Therefore, it is difficult to set a resistance of the control coil at a target value. In other words, when many glow plugs are produced in mass production, resistance values of control coils are widely distributed around the target value. In this case, the control coil is excessively or insufficiently heated, so that the temperature of a heating coil cannot correctly be controlled.

In contrast, in this embodiment, the case 4 is formed to have the middle diameter portion 42 in addition to the smaller diameter portion 41 and the larger diameter portion 43, so that a difference in diameter between the portions 41 and 42 becomes small. In this case, even when the control coil 3 is placed in both the portions 41 and 42 (A2<L1+L2), a compressive force added to a portion of the control coil 3 during the formation of the smaller diameter portion 41 becomes small. Therefore, a wire forming the portion of the control coil 3 is hardly thinned, so that the resistance of the control coil 3 can reliably have a value as originally set. Accordingly, the control coil 3 can be heated as designed, so that the temperature of the heating coil 2 can correctly be controlled.

Furthermore, the whole control coil 3 is placed in the smaller diameter portion 41. In other words, a sum of the lengths L1 and L2 of the coils 2 and 3 is set to be equal to or smaller than the length A2 of the portion 41. Therefore, the whole control coil 3 receives a small compressive force during the formation of the portion 41 by swaging, so that the control coil 3 can reliably have a resistance originally designed. Accordingly, the temperature of the heating coil 2 can further correctly be controlled.

As still another feature of the plug 1, a wire of each of the coils 2 and 3 is set to have a large diameter ranging from 0.31 to 0.35 mm, as compared with a diameter of a wire in the conventional glow plug. Therefore, a current density (or density in electric power) in each of the coils 2 and 3 can be lessened. Accordingly, durability of the coils 2 and 3 can be further improved.

Accordingly, the temperature of the glow plug 1 can further easily be controlled, and the glow plug 1 can further have an excellent durability at a high temperature.

In this embodiment, the heater case 4 of the plug 1 has the three portions 41, 42 and 43 having different diameters. However, the case 4 may have four or more portions having different diameters such that the diameter of the case 4 is made small toward the front side.

Further, in this embodiment, the glow plug 1 heats an air-fuel mixture filled in a combustion chamber of the engine. However, the plug 1 may heat any heated medium disposed in an arbitrary chamber.

COMPARATIVE EXAMPLE

A conventional glow plug is shown in FIG. 4 and FIG. 5 as a comparative example. FIG. 4 is a perspective front view of a front portion of a conventional glow plug, while FIG. 5 is a front view of the front portion of the glow plug shown in FIG. 4. As shown in FIG. 4, a conventional glow plug 9 has a heating coil 92 made of Ni—Cr alloy, a control coil 93 made of Co—Fe alloy, a heater case 94 made of stainless steel, and a housing 95. A length L11 of the coil 92 along an axial direction of the plug 9 is equal to 10 mm. A length L12 of the coil 93 along the axial direction is equal to 24 mm so as to be longer than the length of the coil 3. A wire forming each of the coils 92 and 93 has a diameter of 0.26 mm.

As shown in FIG. 5, the case 94 has a smaller diameter portion 941 on a front side thereof, a larger diameter portion 943 on a rear side thereof, and a taper portion 944 between the portions 941 and 943. Each of the portions 941 and 943 is formed in a cylindrical shape. A diameter D11 of the portion 941 is equal to 3.5 mm. A diameter D13 of the portion 943 is equal to 5.0 mm. A length A11 of the case 94 along the axial direction ranges from 30 to 36 mm. A length A12 of the portion 941 along the axial direction is equal to 26 mm. Therefore, a sum of the lengths L11 and L12 is larger than the length A12. In this case, as shown in FIG. 4, the control coil 93 is placed in the portions 941, 944 and 943. That is, a portion of the control coil 93 is placed in the taper portion 944, and a rear end portion 931 of the control coil 93 is placed in the larger diameter portion 943.

Other structures of the plug 9 are the same as those of the plug 1.

In this comparative example, because of the heating coil 92 made of the Ni—Cr alloy and the heater case 94 made of the stainless steel, the plug 9 is difficult to sufficiently have durability in the circumference of a high temperature reachable according to the PWM current control. The control coil 93 made of the Co—Fe alloy is difficult to be produced at a low cost.

Further, the control coil 93 has the long length L12 so as to reach an attaching area between the case 94 and the housing 95. When carbon particulate matters are deposited on an outer wall of the case 94, a current of the coil 93 cannot be accurately controlled. More specifically, an opening of a plug hole not occupied by the plug 9 is stuffed or packed with carbon particulate matters, and the carbon particulate matters are easily deposited on an outer wall of the case 94. In this case, heat of the coil 93 is easily dissipated through the carbon particulate matters, so that a predetermined relation between resistance of the coil 93 and electric power supplied to the coil 93 is not satisfied. Therefore, an electric current supplied to the heating coil 92 cannot be adequately controlled, and the heating coil 92 is sometimes disconnected or broken.

Moreover, the case 4 has only two portions 941 and 943 of different diameters in two stages so as to have a large difference in diameter between the portions 941 and 943, and the control coil 94 is placed in both the portions 941 and 943. Therefore, resistance of the control coil 93 is easily shifted from a value originally designed by swaging at a high probability, so that the temperature of the heating coil 94 cannot adequately be controlled.

Furthermore, each of the coils 92 and 93 has a small diameter of 0.26 mm. Therefore, a current density or power density in each of the coils 93 and 94 becomes large. In this case, when a large amount of electric current flows through the coils 92 and 93, there is a high probability that durability of the coils 92 and 93 may be lowered.

Experiment 1

The inventor of this application took a first durability test for samples of a glow plug and has ascertained that a heating coil made of Fe—Cr—Al alloy is superior in durability at a high temperature, as compared with durability of a heating coil made of Ni—Cr alloy.

First samples of the glow plug 1 were prepared as experimental examples according to the embodiment shown in FIG. 1 to FIG. 3. Each first sample has a heating coil 2 made of Fe—Cr—Al alloy. Second samples of a glow plug were prepared as comparative examples. Each second sample has a heating coil made of Ni—Cr alloy. Each of the heating coils of the first and second samples is made of a wire of which a diameter is set at 0.31 mm according to this embodiment. A heater case in each of the first and second samples is made of SUS310 which represents a stainless steel and is specified by Japanese Industrial standards. SUS310 contains nickel of 20 wt % and chromium of 25 wt % and slightly contains carbon, silicon, manganese, phosphorous and silicon. The other structures in each of the first and second samples are the same as those of the glow plug 1 according to this embodiment.

In this durability test, a controlled current is supplied to each sample in an on-off cycle set at one minute until a heating coil of the sample is broken. The number of cycles required for the breakage of the heating coil is called a durability cycle number in this application. As a voltage of the current, three values are adopted to saturate heating coils of three samples at a first maximum temperature of 950° C., a second maximum temperature of 1000° C. and a third maximum temperature of 1050° C., respectively. A durability test is performed for each of the maximum temperatures.

Experimental results for the samples are shown in FIG. 6. FIG. 6 shows a relation between durability of a heating coil and a maximum temperature of the heating coil. In FIG. 6, an experimental result of each first sample having a heating coil 2 made of Fe—Cr—Al alloy is indicated by a sign Δ, and an experimental result of each second sample having a heating coil made of Ni—Cr alloy is indicated by a sign ◯. Two first samples and two second samples are heated to the second maximum temperature of 1000° C.

As shown in FIG. 6, it will be realized that, as the maximum temperature is increased, a durability cycle number is decreased. For example, at the maximum temperature of 1000° C., the heating coil made of Ni—Cr alloy is broken before the durability cycle number reaches 10000 cycles. In contrast, the heating coil made of Fe—Cr—Al alloy is not broken until the durability cycle number largely exceeds 10000 cycles.

Accordingly, a heat resisting property of the heating coil 2 made of the Fe—Cr—Al alloy according to this embodiment can be improved as compared with that of a heating coil made of the Ni—Cr alloy, and the glow plug 1 with the heating coil 2 made of the Fe—Cr—Al alloy can reliably have an excellent durability at a high temperature such as 1000° C.

Experiment 2

The inventor took a second durability test for samples of a glow plug and has ascertained, based on experiments, that a heater case made of an inconel material is superior in durability at a high temperature.

First samples of the glow plug 1 were prepared as experimental examples according to the embodiment shown in FIG. 1 to FIG. 3. Each first sample has a heater case 4 made of an inconel material. Second samples of a glow plug were prepared as comparative examples. Each second sample has a heater case made of SUS310 representing stainless steel. Each of the first and second samples has a heating coil 2 made of Fe—Cr—Al alloy and a control coil 3 made of nickel according to the embodiment. Each of the heating coils 2 of the first and second samples is made of a wire of which a diameter is set at 0.31 mm according to the embodiment. The other structures in each of the first and second samples are the same as those of the glow plug 1.

This durability test is made in the same manner as in the first durability test.

Experimental results for the samples are shown in FIG. 7. FIG. 7 shows a relation between durability of a heating coil and a maximum temperature of the heating coil. In FIG. 7, an experimental result of each first sample having a heater case 4 made of the inconel material is indicated by a sign □, and an experimental result of each second sample having a heater case made of SUS310 is indicated by a sign Δ.

As shown in FIG. 7, it will be realized that, as the maximum temperature is increased, a durability cycle number is decreased. For example, at the maximum temperature of 1050° C., the heating coil accommodated in the heater case made of the stainless steel is broken before the durability cycle number reaches 10000 cycles. In contrast, the heating coil accommodated in the heater case 4 made of the inconel material is not broken until the durability cycle number largely exceeds 10000 cycles.

Accordingly, a heat resisting property of the glow plug 1 having the heater case 4 made of the inconel material according to this embodiment can be improved as compared with that of the glow plug having the heater case made of the stainless steel, and the glow plug 1 with the heater case 4 made of the inconel material can reliably have an excellent durability at a high temperature such as 1050° C.

Experiment 3

The inventor took a coil length determining test for the glow plug 1 and experimentally determined a range of the length L2 of the control coil 3 required to heighten the maximum temperature of the heating coil 2 to 1000° C.

Many samples of a glow plug were prepared such that the control coils of the samples have various lengths including a range from 5 to 12 mm. The heating coil 2 of each sample was made of a wire of which a diameter was set at 0.31 mm according to the embodiment. The length L1 of the heating coil 2 of each sample was set at 10 mm. Other structures of the samples are the same as those of the glow plug 1 according to the embodiment.

In this test, the duty ratio in the controlled current is set at 100% in the PWM current control, and a rated voltage of the controlled current applied to the coils 2 and 3 is set at 4.7 V. The applied voltage of the controlled current is inevitably fluctuated during the operation of the plug 1 by a voltage change α ranging from −0.3 to +0.3 V. Therefore, in addition to an experiment set at the applied voltage of 4.7 V, an experiment set at the applied voltage of 4.4 V (4.7−α) and an experiment set at the applied voltage of 5.0 V (4.7+α) were made.

FIG. 8 shows a relation between a length of a control coil and a maximum temperature of a heating coil for each of three values of the applied voltage. In FIG. 8, each experimental result corresponding to an applied voltage set at 4.7 V is indicated by a sign ◯ indicates an experimental result obtained when an applied voltage of the controlled current is set at the rated voltage of 4.7 V. Each experimental result corresponding to an applied voltage set at 4.4 V is indicated by a sign Δ. Each experimental result corresponding to an applied voltage set at 5.0 V is indicated by a sign □.

As shown in FIG. 8, it will be realized that, when the length L2 of the control coil 3 is set within a range from 5 to 12 mm, the heating coil 2 is heated to the maximum temperature of 1000° C. even when the applied voltage is fluctuated around the rated voltage. Further, under the PWM current control, when the length L2 of the control coil 3 is shorter than 5 mm, an electric current excessively flows through the heating coil 2. Therefore, the temperature of the heating coil 2 is excessively heightened, and durability of the heating coil 2 maybe lowered. In contrast, when the length L2 of the control coil 3 is longer than 12 mm, an electric current insufficiently flows through the heating coil 2. Therefore, the temperature of the heating coil 2 is insufficiently heightened, so that combustion efficiency in the engine may be deteriorated.

Accordingly, when the length L2 of the control coil 3 is set within a range from 5 to 12 mm, the heating coil 2 can have an excellent durability and can be heated appropriately.

Experiment 4

The inventor examined the influence of carbon matters deposited on a heater case of a glow plug and has ascertained that the control coil 3 set at the length of 12 mm is appropriate to heat an air-fuel mixture without receiving adverse any influences from carbon particulate matters.

A plurality of glow plugs 1 shown in FIG. 1 and FIG. 2 were prepared. Each plug 1 has the control coil 3 set at the length L2 of 12 mm. A plurality of glow plugs 9 shown in FIG. 4 and FIG. 5 were prepared. Each plug 9 has the control coil 93 set at the length L12 of 24 mm. Further, each of the plugs 1 has the heating coil 2 set at the length L1 of 10 mm and the case 4 set at the length A1 of 36 mm. each of the plugs 9 has the heating coil 92 set at the length L11 of 10 mm and the case 94 set at the length A11 of 36 mm. A wire of each of the coils 2, 3, 92 and 93 has a diameter of 0.31 mm.

FIG. 9 shows a relation between a length of packed carbon particulate matters and the maximum temperature of the heating coil 2 for each of the lengths L2 and L12 of a control coil. FIG. 10 is a sectional view of a plug hole penetrating through a wall of a combustion chamber. Each of FIG. 11A, FIG. 11B, FIG. 11C, FIG. 12A, FIG. 12B and FIG. 12C is an explanatory view showing carbon particulate matters packed in the plug hole receiving a glow plug. FIG. 11A shows the carbon particulate matters deposited by a length of 10 mm along the axial direction, FIG. 11B shows the carbon particulate matters deposited by a length of 15 mm along the axial direction, and FIG. 11C shows the carbon particulate matters deposited by a length of 20 mm along the axial direction. FIG. 12A shows the carbon particulate matters deposited by a length of 25 mm along the axial direction, FIG. 12B shows the carbon particulate matters deposited by a length of 30 mm along the axial direction, and FIG. 12C shows the carbon particulate matters deposited by a length of 33 mm along the axial direction.

As shown in FIG. 11A, FIG. 11B, FIG. 11C, FIG. 12A, FIG. 12B and FIG. 12C, each of the glow plugs 1 and 9 was inserted into the plug hole 6, and carbon particulate matters C were forcibly deposited on the case 4 or 94 of the glow plug as experiments. An amount of the deposited carbon particulate matters was adjusted to obtain invention samples having the carbon particulate matters deposited on the cases 4 of the plugs 1 by a length of 10 mm, a length of 15 mm, a length of 20 mm, a length of 25 mm, a length of 30 mm and a length of 33 mm, respectively. Further, in the same manner, comparative samples were obtained so as to have the carbon particulate matters deposited on the cases 94 of the plugs 9 by a length of 10 mm, a length of 15 mm, a length of 20 mm, a length of 25 mm, a length of 30 mm and a length of 33 mm, respectively.

Then, a voltage set at 4.7 V was applied to the coils of each sample for one minute such that the heating coil reaches its maximum temperature. The temperature of the heating coil in each sample was measured. Measured results are shown in FIG. 9. In FIG. 9, measured results of the invention samples are indicated by signs ⋄ and ♦, and measured results of the comparative samples are indicated by signs ◯ and  indicate. The samples having no packed carbon particulate matters are indicated by the signs ⋄ and ◯.

As shown in FIG. 9, the heating coil 92 of the plug 9 is largely heightened to the temperature higher than 1050° C. when the carbon particulate matters extend from the front end of the housing 95 towards the front side by the length ranging from 15 mm to 25 mm. In contrast, the heating coil 2 of the plug 1 is slightly heightened to the temperature ranging between 1000° C. and 1050° C. when the carbon particulate matters extend from the front end 51 of the housing 5 towards the front side by the length ranging from 15 mm to 25 mm.

Accordingly, the glow plug 1 according to this embodiment can suppress an excessive heating of the heating coil 2 so as to have an excellent durability, and an air-fuel mixture can be adequately heated.

Further, when the length of the carbon particulate matters reaches 30 mm or more, the temperatures of the heating coils 2 and 92 are lowered. The reason is as follows. The carbon particulate matters extending by the length of 30 mm or more reach a portion of the heater case surrounding the heating coil, and heat of the heating coil is dissipated to the wall of the chamber through the carbon particulate matters. Therefore, the temperatures of the heating coils 2 and 92 are lowered. However, in an actual operation of the engine, the length of the carbon particulate matters does not reach 30 mm.

Experiment 5

The inventor analyzed influence of swaging on the resistance of a control coil by comparing a distribution of resistance values of samples of the control coil 3 and a distribution of resistance values of samples of the control coil 93.

More specifically, ten first inventive samples of the glow plug 1 shown in FIG. 1 to FIG. 3 were prepared. Each first inventive sample has the heating coil 2 set at the length L1 of 10 mm, the control coil 3 set at the length L2 of 12 mm, and the smaller diameter portion 41 set at the length A2 of 24 mm. Because the length A2 is longer than a sum of L1 and L2, the whole control coil 3 is placed in the smaller diameter portion 41. Ten second inventive samples of the glow plug 1 were prepared. Each second inventive sample has the heating coil 2 set at the length L1 of 10 mm, the control coil 3 set at the length L2 of 12 mm, and the smaller diameter portion 41 set at the length A2 of 16 mm. Because the length A2 is shorter than a sum of L1 and L2, a major portion of the control coil 3 is placed in the smaller diameter portion 41, and a rear portion of the control coil 3 is placed in the first taper portion 441. Ten comparative samples of the glow plug 9 shown in FIG. 4 and FIG. 5 were prepared. Each comparative sample has the heating coil 92 set at the length L11 of 10 mm, the control coil 93 set at the length L12 of 24 mm, and the smaller diameter portion 941 set at the length A12 of 26 mm. Because the length A12 is shorter than a sum of L11 and L12, a major portion of the control coil 93 is placed in the smaller diameter portion 941, and a rear portion of the control coil 93 is placed in the first taper portion 944 and the larger diameter portion 943.

Sizes of the first inventive samples, the second inventive samples and the comparative samples are shown in Table 1.

TABLE 1 first second comparative inventive inventive samples samples samples L11 10 L1 10 10 L12 24 L2 12 12 A12 26 A2 24 16 D11 3.5 D1 3.5 3.5 D2 4 4 D13 5 D3 5 5 (unit: mm)

The resistance of the control coil in each sample was measured. Measured resistance values of the control coils in the samples are shown in FIG. 13.

As shown in FIG. 13, it will be realized that resistance values of the control coils 93 in the comparative samples are widely distributed around a target value Rt in a range from (1−0.05)Rt to (1+0.05)Rt. In contrast, resistance values of the control coils 3 in the first and second invention samples are narrowly distributed around the target value Rt in a range from (1−0.04)Rt to (1+0.04)Rt.

Accordingly, a change in resistance of the control coil 3 can be reduced in the glow plug 1 even when a portion of the control coil 3 is placed in the first taper portion 441, so that the heating coil 2 can have an excellent durability. The reason the change is reduced in the glow plug 1 is as follows. The glow plug 1 has the middle diameter portion 42 in addition to the portions 41 and 43, so that a difference between D1 and D2 is set to be small. In this case, when the case 4 is formed by swaging, no strong external force is added to the control coil 3. Therefore, a wire forming the control coil 3 is not thinned, so that the resistance of the control coil 3 is hardly changed by swaging even when a portion of the control coil 3 is placed in the first taper portion 441.

Experiment 6

The inventor took a third durability test while changing a diameter of a wire forming the heating coil 2 and has ascertained that the heating coil 2 formed by a wire having a diameter of 0.31 mm or more has an excellent durability.

A plurality of first invention samples of the grow plug 1 shown in FIG. 1 and FIG. 2, a plurality of second invention samples of the grow plug 1 shown in FIG. 1 and FIG. 2 and a plurality of comparative samples of the grow plug 9 shown in FIG. 4 and FIG. 5 were prepared. Each first invention sample has a heating coil 2 formed of a wire having a diameter of 0.31 mm. Each second invention sample has a heating coil 2 formed of a wire having a diameter of 0.35 mm. Each comparative sample has a heating coil 92 formed of a wire having a diameter of 0.26 mm. The heating coil of each sample is made of Fe—Cr—Al alloy. A heater case of each sample is made of SUS 310 representing stainless steel. A control coil of each sample is made of Ni and is formed of a wire having a diameter of 0.31 mm.

This durability test is made in the same manner as in the first durability test.

Experimental results for the samples are shown in FIG. 14. FIG. 14 shows a relation between durability of the heating coil and a maximum temperature of the heating coil. In FIG. 14, experimental results for the comparative samples (diameter of 0.26 mm) are indicated by signs □, experimental results for the first invention samples (diameter of 0.31 mm) are indicated by signs , and experimental results for the second invention samples (diameter of 0.35 mm) are indicated by signs ∇.

As shown in FIG. 14, it will be realized that, as the diameter of a wire forming the heating coil is increased, the durability of the heating coil is heightened. When the diameter of a wire forming the heating coil is set to be equal to or larger than 0.31 mm, the durability cycle number largely exceeds 10000 at the maximum temperature of 1000° C., and the heating coil has an excellent durability.

Accordingly, the heating coil 2 according to this embodiment can have an excellent durability at a high temperature such as 1000° C.

Experiment 7

The inventor took a fourth durability test while changing a diameter of a wire forming the control coil 3 and has ascertained that, when the control coil 3 is formed by a wire having a diameter of 0.31 mm or more, the heating coil 2 has an excellent durability.

A plurality of first invention samples of the grow plug 1 shown in FIG. 1 and FIG. 2, a plurality of second invention samples of the grow plug 1 shown in FIG. 1 and FIG. 2 and a plurality of comparative samples of the grow plug 9 shown in FIG. 4 and FIG. 5 were prepared. Each first invention sample has a control coil 3 formed of a wire having a diameter of 0.31 mm. Each second invention sample has a control coil 3 formed of a wire having a diameter of 0.35 mm. Each comparative sample has a control coil 93 formed of a wire having a diameter of 0.26 mm. The control coil of each sample is made of Ni. The heating coil of each sample is made of Fe—Cr—Al alloy and is formed of a wire having a diameter of 0.31 mm. A heater case of each sample is made of SUS 310 representing stainless steel.

This durability test is made in the same manner as in the first durability test.

Experimental results for the samples are shown in FIG. 15. FIG. 15 shows a relation between durability of the heating coil and a maximum temperature of the heating coil. In FIG. 15, experimental results for the comparative samples (diameter of 0.26 mm) are indicated by signs □, experimental results for the first invention samples (diameter of 0.31 mm) are indicated by signs Δ, and experimental results for the second invention samples (diameter of 0.35 mm) are indicated by signs .

As shown in FIG. 15, as the diameter of a wire forming the control coil is increased, the durability of the heating coil is heightened. When the diameter of a wire forming the control coil is set to be equal to or larger than 0.31 mm, the durability cycle number largely exceeds 10000 at the maximum temperature of 1000° C., and the heating coil has an excellent durability.

Accordingly, the heating coil 2 according to this embodiment can have an excellent durability at a high temperature such as 1000° C. 

1. A glow plug comprising: a heating coil made of an alloy of iron, chromium and aluminum; a control coil made of nickel and serially connected with the heating coil; a current supply line through which an electric current adjusted according to a pulse width modulation control is supplied to the control coil and the heating coil to heat the heating coil to a maximum temperature while heightening a resistance of the control coil; and a heater case, made of an inconel material, that accommodates the heating coil and the control coil so as to dispose the control coil on a rear side of the heating coil and insulates the heating coil and the control coil from a heated medium disposed outside the heater case, the heated medium being heated by the heating coil through the heater case.
 2. The glow plug according to claim 1, wherein a length of the control coil along an axial direction of the heater case is set in a range from 5 to 12 mm.
 3. The glow plug according to claim 1, further comprising a housing that holds a rear end of the heater case, wherein a sum of the length of the control coil and a length of the heating coil along the axial direction is smaller than a length of the heater case such that the control coil is placed out of a rear portion of the heater case facing the housing.
 4. The glow plug according to claim 1, wherein the heater case has three or more cylindrical portions having different diameters such that the diameter of the heater case is made small toward a front side opposite to the rear side.
 5. The glow plug according to claim 4, wherein the control coil is disposed only in the cylindrical portion which has the smallest diameter among those of the cylindrical portions and is disposed on the front side.
 6. The glow plug according to claim 4, wherein the heater case further has a taper portion disposed between one cylindrical portion, having the smallest diameter among those of the cylindrical portions and disposed on the front side, and another one of the cylindrical portions adjacent to the one cylindrical portion having the smallest diameter, and the control coil is disposed only in both the cylindrical portion having the smallest diameter and the taper portion.
 7. The glow plug according to claim 1, wherein each of the heating and control coils is made of a wire of which a diameter is set within a range from 0.31 to 0.35 mm. 